1. Field of the Invention
The present invention relates to an inverter for a motor for driving an electric vehicle, and particularly relates to an input resistance control system of power elements (such as IGBT (Isolated Gate Bipolar Transistors)) for the purpose of reducing switching loss of the electric power elements.
2. Background Art
In general, a motor (e.g. a synchronous motor) composed of a rotor having a plurality of pairs of poles and a stator constituted by a three-phase winding is used for driving, electric vehicles and a predetermined driving force is obtained by supplying an alternating current power generated by an inverter based on a predetermined command current to the three-phase winding of a stator.
The above-described inverter is formed, as shown in FIG. 10, by connecting, using a three arm bridge, six power elements (such as IGBT elements, insulating gates, and bipolar transistors, and hereinafter, the power elements are assumed to be IGBT elements), and the alternating current power for driving the motor is obtained from the direct current source connected to current carrying devices by controlling the current carrying devices by gate controlling and switching these power elements.
Inputting a predetermined driving pulse to the gate of the IGBT elements carries out switching of IGBT elements.
There are two methods for driving the gate. The first method is to turn OFF the IGBT element 4 while maintaining the IGBT element 1 ON, as shown in FIG. 11, in the case of stopping a current i1 which flows in the motor winding, as shown in FIG. 10, by supplying the current i1 through the positive side IGBT element 1 connected to one arm and the negative side IGBT element 4 connected to the other arm. At this time, the current flowing in the motor constituting an inductive load flows to the diode portion by passing the IGBT element 1 and IGBT element 3 through a current route i11, and the above IGBT element 1 is turned OFF after the current i11 has been attenuated.
The second method for driving the gate is, as shown in FIG. 12, a driving method to turn OFF the IGBT element 1 and IGBT element 4 simultaneously. At this time, since the current i1, shown in FIG. 10, flowing in the motor M is fed back as a current i12 passing through the diode portion of the IGBT element 2 and the diode portion of the IGBT 3, the current i12 disappears within a comparatively short period of time.
However, when the gate is driven as described above, the switching speed is delayed by blunting the rise and fall of the signal voltage applied to the gate using a parasitic capacitance originated between the gate and emitter of the IGBT element and the input resistance, which is attached in series to the gate of the IGBT element, in order to prevent breakdown of the elements by a surge voltage.
When the switching speed is reduced, the problem arises that the delayed switching generates heat in the IGBT element due to the switching loss, and the generated heat causes thermal breakdown of the elements and, in contrast, requires a large cooling device. The problem also arises because the increase of the heat loss and the large cooling system consumes the electric power of the battery, which results in reducing the traveling distance of the vehicle.
It is therefore an object of the present invention to provide a control apparatus for electric vehicles, which is capable of reducing the switching loss while restricting the surge voltage to a safe withstand voltage below an allowable surge withstand voltage by controlling the resistance of an input resistance depending on the traveling state of the electric vehicle.
According to the first aspect of the present invention, a control apparatus for an electric vehicle is provided, which controls a traveling electric vehicle by supplying an electric power to a motor for driving the vehicle through an inverter comprising an input resistance value setting device for setting the resistance value of an input resistor to a plurality of electric power elements in said inverter in response to the driving state of the vehicle.
In the control apparatus for an electric vehicle according to the first aspect, the input resistance value setting device sets said resistance value in response to any one of said battery voltage, the temperature of said electric power elements, or the power consumption of said power elements.